The subject of the invention is a gas discharge lamp having a double electrode arrangement in which the discharge space is formed by a glass tube which is subdivided approximately in the center with a separation plate along the largest part of its length.
Mercury vapor low pressure discharge lamps generally have a discharge space which is formed by a glass tube. The length of the discharge path is therefore equal to the length of the tube, limited practically by its handling and mechanical sensitivity. In order to lengthen the discharge path without great handling problems, U-shaped, W-shaped or meandrous tube shapes were suggested and also employed for special uses in illumination technology.
The form of the discharge tube in gas discharge lamps for d.c. current operation is of particular importance. There, particularly in lamps with discharge paths over about 60cm (at a tube diameter of 37 mm), the appearance of a cataphoresis occurs, i.e., a concentration of the metal vapor in the cathode volume, which leads to an uneven light output and a decreased light efficiency. In order to avoid this effect, for example, the German Pat. Nos. 2,254,709 and 2,319,401 suggested lamp constructions having a so-called double electrode arrangement in which the electrodes are closely adjacent. The two electrode spaces are gas or metal vapor permeably connected with one another but are also connected with one another in discharge-tight fashion. These German patents show that the discharge volume is enclosed by a glass tube shaped into a closed or sealed form whose cross section is interrupted at one location in discharge-tight fashion, but, however, is gas or metal vapor permeably interrupted or narrowed down such that the electrodes are arranged at both sides close to this interruption or narrowing down location.
With the aid of such an embodiment, the discharge path for equal constuction lengths of the lamp is at least doubled and the effects of cataphoresis for d.c. current operation is eliminated. However, the embodiment having a double electrode arrangement is considerably more expensive in production. Aside from the requirement of the tube bending or the joining of a tube consisting of several individual parts, this expense holds true primarily for the electrode arrangement per se, because the narrowing-down or the gas-permeable interruption of the discharge space must be situated between the electrodes, the feed-lines of the electrodes must be conveyed through the tube wall in gas-tight fashion and, moreover, at least one opening, preferably in the form of a pump tube for the evacuation and filling of the discharge lamp, must be attached.
The German application P 26 56 776.0 provides a simple production arrangement of the components in the electrode region which can be relatively simply realized, utilizing a solder glass technique. However, this construction also contains the basic difficulty that a discharge space has to be formed from a correspondingly bent tube while inserting an electrode part. This discharge space must be closed at all sides and gas-tight with respect to the exterior.